using System.Security.Cryptography;
using System;

public class Crc32 : HashAlgorithm
{
	public const UInt32 DefaultPolynomial = 0xedb88320;
	public const UInt32 DefaultSeed = 0xffffffff;

	private UInt32 hash;
	private UInt32 seed;
	private UInt32[] table;
	private static UInt32[] defaultTable;

	public Crc32()
	{
		table = InitializeTable(DefaultPolynomial);
		seed = DefaultSeed;
		Initialize();
	}

	public Crc32(UInt32 polynomial, UInt32 seed)
	{
		table = InitializeTable(polynomial);
		this.seed = seed;
		Initialize();
	}

	public override void Initialize()
	{
		hash = seed;
	}

	protected override void HashCore(byte[] buffer, int start, int length)
	{
		hash = CalculateHash(table, hash, buffer, start, length);
	}

	protected override byte[] HashFinal()
	{
		byte[] hashBuffer = UInt32ToBigEndianBytes(~hash);
		this.HashValue = hashBuffer;
		return hashBuffer;
	}

	public override int HashSize
	{
		get { return 32; }
	}

	public static UInt32 Compute(byte[] buffer)
	{
		return ~CalculateHash(InitializeTable(DefaultPolynomial), DefaultSeed, buffer, 0, buffer.Length);
	}

	public static UInt32 Compute(UInt32 seed, byte[] buffer)
	{
		return ~CalculateHash(InitializeTable(DefaultPolynomial), seed, buffer, 0, buffer.Length);
	}

	public static UInt32 Compute(UInt32 polynomial, UInt32 seed, byte[] buffer)
	{
		return ~CalculateHash(InitializeTable(polynomial), seed, buffer, 0, buffer.Length);
	}

	private static UInt32[] InitializeTable(UInt32 polynomial)
	{
		if (polynomial == DefaultPolynomial && defaultTable != null)
			return defaultTable;

		UInt32[] createTable = new UInt32[256];
		for (int i = 0; i < 256; i++)
		{
			UInt32 entry = (UInt32)i;
			for (int j = 0; j < 8; j++)
				if ((entry & 1) == 1)
					entry = (entry >> 1) ^ polynomial;
				else
					entry = entry >> 1;
			createTable[i] = entry;
		}

		if (polynomial == DefaultPolynomial)
			defaultTable = createTable;

		return createTable;
	}

	private static UInt32 CalculateHash(UInt32[] table, UInt32 seed, byte[] buffer, int start, int size)
	{
		UInt32 crc = seed;
		for (int i = start; i < size; i++)
			unchecked
			{
				crc = (crc >> 8) ^ table[buffer[i] ^ crc & 0xff];
			}
		return crc;
	}

	private byte[] UInt32ToBigEndianBytes(UInt32 x)
	{
		return new byte[] {
			(byte)((x >> 24) & 0xff),
			(byte)((x >> 16) & 0xff),
			(byte)((x >> 8) & 0xff),
			(byte)(x & 0xff)
		};
	}
}